Nothing.
Prior to owning the M3, I'd always looked down on the car as being just another 3-series, albeit with a bit of grunt under your right foot. Now I realise just how frighteningly quick these cars are.
An acquaintance of mine had often described how deceptively fast an M3 is, but I'd never really believed him. Like Subaru Impreza Turbos, there are too many around and you hardly give them a second glance, let alone a second thought. But let me tell you that they are seriously rapid cars, with a steady flow of power grabbing the wheels all the way round the tacho from idle to the red line. All the while it sounds terrific -- harsh to begin with, turning into a banshee wail later on. "Intoxicating" is a cliche but it fits so well.
If this wasn't enough, every M3 is built to the standard everyone expects from BMW, and mine was convertible with a fully automatic power hood -- the 3-Series without doubt makes the best-looking drop-top -- and to top it all was virtually shake free. Only a minor shudder through the wheel down broken roads spoiled the feel. It's also a relative bargain, rating price against performance.
Looking back the only complaint I would make is that the car I owned had no traction control, now standard on the current M3, and was a manual. Although people expect manual boxes in performance cars like this, the SMG option would I think have suited me better as I've never got along with either BMW or Mercedes transmissions requiring user intervention!
I've owned my 1996MY E36 Evo convertible for a year & I'm still enjoying it!
Things gone wrong: sticking hood on occasions, slight water leaks between hood & top of the A-pillars, air conditioning not working (not yet fixed, but who cares when the hood is down?)
Having previously owned E36 M3 coupes (3.0 then 3.2), the most noticeable difference is the inferior ride & handling. It's still good, but the reduced body stiffness is very noticeable. The performance is still great, albeit 9/10ths of the coupe. The dynamic trade off is more than worth it when the hood is down on a hot sunny day! I'm averaging 27MPG, which is good considering it's performance potential. I'm keeping hold of it until E46 M3 prices drop to my level. Overall a great car!